Often times it is necessary to signal to an aircraft from the ground. For example, in military tactical operations, ground troops signal aircraft such as helicopters to identify landing sites, targets, and the like. In that such aircraft often fly night missions, they normally are equipped with heat sensors, such as infrared sensors, by which the aircraft can be flown. For instance, the AH-64 (Apache) helicopter is equipped with forward looking infrared (FLIR) that senses light in the 6-12 micron range.
Despite being so equipped, helicopters normally are landed in night tactical environments without the use of heat sensing systems such as FLIR. In the case of the AH-64 helicopter, a copilot gunner positioned in the front seat of the helicopter images light signals sent from personnel on the ground and, from these light signals, provides verbal instructions to the pilot-in-command seated in the back seat to aide him in landing the aircraft. Such verbal instructions from the copilot gunner generally are necessary in such situations because the pilot-in-command cannot see the immediate foreground of the helicopter. The light signals sent to the helicopter occupants normally are made with a plurality of individual light sources such as light sticks. To aid the copilot gunner in viewing the light signal, the copilot normally uses night vision goggles which amplify the light signals emitted by the light sources.
The signaling procedure described above is lacking for several reasons. First, the pilot-in-command must land the helicopter "blind" in that the light sources used to form the light signal do not emit enough heat so as to be detectable with heat sensing systems such as FLIR. Therefore, the pilot-in-command must rely solely on the verbal instructions given by the copilot gunner seated in the front seat in landing the aircraft. Although the verbal instructions provided by the copilot gunner normally are adequate so as to facilitate proper landing of the helicopter, such a procedure is inferior to a procedure in which the pilot-in-command can also see the signals provided by the ground personnel. Moreover, since the copilot gunner must concentrate on the light signals and on providing precise verbal instructions to the pilot-in-command, the copilot cannot focus on ensuring that the aircraft is clear of all obstructions that could interfere with the aircraft during landing. Therefore, the landing procedure described above provides a substantial risk of crash landings and personnel injuries. In addition to the craft landing disadvantages of conventional signaling systems, use of such systems further invites enemy attack, in that conventional light sources emit light in the visible spectrum. These light signals directed toward friendly aircraft can similarly be seen by enemy troops. Therefore, when such signaling systems are used, the aircraft occupants, as well as ground personnel, are vulnerable to enemy attack.
In an attempt to overcome some of the drawbacks of the aforementioned signaling systems, alternative field solutions have been tried. In one such solution, metal cans, such as coffee cans, have been filled with sand soaked in jet fuel and then ignited so as to burn the jet fuel. Once the fuel had burned for a period of time adequate to heat the metal cans and sand, the flames were extinguished and the cans arranged in an inverted "Y" configuration such that the heat signature of the cans would create a thermal "Y" signal that could be detected by the helicopter heat sensing system. Although providing enough heat so that the heat sensing system could be utilized, this signaling system also is disadvantageous for several reasons. First, jet fuel is highly flammable. For this reason, its use creates unacceptable safety risks to ground personnel and aircraft occupants alike. Secondly, the heated cans provide non-directional heat signals that can be detected by enemy forces as well as the aircraft pilots. Therefore, the use of such heat signaling devices again risks enemy attacks.
From the above, it can be appreciated that it would be advantageous to have a thermal signaling system that is detectable by a heat sensing systems such as FLIR which further is generally safe to use and which is not easily detectable to enemy troops.